The invention relates to an improved process intended to optimize the magnetic properties of a material provided with permanent magnet properties, with a view to obtaining a product which has high magnetic performance and is in finely divided form. More specifically, it relates to a process suitable for increasing the internal magnetic energy of such a material, of the rare earth/iron/boron alloy type, obtained after decrepitation by a hydrogenation/dehydrogenation procedure. Finally, it also relates to the products obtained by this process.
2. Description of the Prior Art
The need for materials which have high magnetic properties and are in pulverulent form is becoming greater every day, in particular within the framework of their use for the production of sintered or bonded magnets (injected or pressed).
The production of bonded magnets is fundamentally carried out by introducing a large amount of magnetic material in the most finely divided form possible into a continuous organic matrix, generally produced from a synthetic polymer. This step is conventionally carried out using a twin screw, at the melting point of the polymer. In this way, in order to obtain high performance bonded magnets, the aim is to introduce the largest possible amount of magnetic material into the matrix. Within the framework of an optimization of such magnets, the intended aim is to minimize the size of the constituent "particles" of the magnetic material, while improving the magnetic properties and in particular the coercivity of the said "particles". In addition, it is important that the particle size distribution of these "particles" is as narrow as possible, in order, in particular, to optimize the magnetic properties (coercivity, induction) of the bonded magnet.
This property proves particularly important in the context of the production of bonded magnets of high anistropy. In fact, on the one hand the dispersibility of the powders, that is to say their ability to disperse homogeneously, for example in the coating matrix or resin, and, on the other hand, their orientability, that is to say their ability to orient under a magnetic field, and more precisely to align their direction of easy magnetization with the direction of the magnetic field applied, and to do so by mechanical rotation, depend on this narrow distribution and on the effective size of the "particles" obtained.
The interest in obtaining powders which have high magnetic properties and are of homogeneous low particle size is thus clearly apparent. Bonded anisotropic magnets produced from these powders are capable of displaying a very much higher remanent induction than those currently produced using the powders available today, this being achieved from the same "absolute" amount of magnetic material. One of the aims of the present invention is to provide a process suitable for the production of such powders, having a high coercivity.
With regard to the production of magnetic materials which are in pulverulent form, a process termed a "decrepitation" process is described in European patent EP-A-O 173 588, which process comprises at least one hydrogenation/dehydrogenation cycle of a compound having the general formula R.sub.2 M.sub.14 B, where B denotes boron and R denotes an element belonging to the group of the rare earths or yttrium, M essentially denoting iron and being able to be partially replaced by other metal elements, enabling a product to be obtained which is of small particle size and has valuable magnetic properties in respect of its magnetization, but generally having an inadequate degree of coercivity.
In fact, it has been possible to show that the magnetic properties of a material were, inter alia, linked to the definition of its microstructure (domain, grain, structure, etc. . . . ). In this way, it has been sought to obtain finely divided materials composed of particles of small size, typically ten micrometers and less, by a process which is easy to carry out and inexpensive and, additionally, without risk for experimenters and producers. This process consists in producing powdering of the base material by hydrogenation. This process, which is termed decrepitation, briefly comprises the absorption of hydrogen by a metal alloy under specific pressure and temperature conditions. This absorption, for which the chemical reaction of binding atomic hydrogen to particular sites of the material gives rise to an evolution of heat, causes an increase in the volume of the alloy, subsequent to the expansion of the crystal lattice, in fact resulting in a dislocation of the solid material. This takes place at two levels, that is to say at an intergranular level, that is to say between the entities of the same phase, and at an intragranular level, corresponding to "bursting" of entities of a given phase. A powder is obtained in agglomerate form, which it is possible to disperse simply by stirring.
It has been found that although the products obtained certainly have a particle size which is completely in accord with the intended aim, on the other hand the magnetic properties obtained must be optimized for some particular applications, in particular for permanent magnets, or for bonded magnets. In fact, the powders obtained certainly have a particle size close to 10 micrometers, or even less; on the other hand, the internal magnetic energy (HB)max developed by said powders remains moderate and in every case is insufficient for the powders to be used as such in the production of permanent magnet/polymers of high capacity.